With a combined gamma-IFN and NGF treatment of a human neuroblastoma cell line grown in a monolayer we established earlier a well differentiated and long surviving variant of human neuroblastoma cells. This system was to serve as an in-vitro model for the in-vivo situation with poliovirus infection of neurons. We found that, contrary to undifferentiated (untreated) cultures of the same cells and non-neural, AGMK, Vero cells, Hep-2 cells, in the differentiated variant replication of poliovirus is suppressed, and selection of neurovirulent revertants significantly enhanced. These findings apparently coincide with a few known observations about the fate of poliovirus in the spinal cord of monkeys used in the monkey test, as well as with our preliminary data obtained by the MAPREC test on virus reversion in the samples of the monkey spinal cords. In addition, we discovered that gamma-IFN efficiently suppresses the replication of poliovirus, a finding not found in the literature.